1. Field of the Invention
The present invention relates to a driver circuit for light sources, in particular LEDs, for a vehicle light.
2. Description of the Related Art
In some conditions of use of a driver circuit of lighting sources, it may happen that the power supply voltage of the circuit undergoes significant variations, and in particular falls well below the nominal value. In this case, if several light sources are connected in series with each other, it may then happen that the power supply voltage is insufficient to guarantee the correct lighting of all the sources.
Such a situation of a drop in the power supply voltage occurs for example when a vehicle turns off automatically when at a standstill, for example, at a traffic light, to then start again when the accelerator is pressed, using the system known as “Start and Stop”. For example, during “Start and Stop”, the power supply voltage may fall from a nominal value of 13.2 volts to 6.0 volts in the worst cases. Even in these operating conditions the vehicle light is required to have as little light fluctuation as possible. This means, that if an LED has a typical junction voltage of 2.5 volts, more than two LEDs cannot be connected in series with each other. Considering in fact the various physiological voltage drops of the circuit, the presence of an anti-inversion diode in input and the current regulation circuit, to drive three LEDs in series at least 9 volts would be needed. Under 9 volts, the luminosity begins to fall and, when the vehicle is stopped at the traffic lights and then starts again, the flickering of the LEDs may be noted.
In the driver circuits for light sources, in particular LEDs, normally used, the lighting sources are positioned in matrixes or in lighting branches, or in combinations thereof. An LED matrix is understood to mean a plurality of LEDs connected in a matrix, that is to say positioned in rows and columns, where the LEDs of each row are connected in parallel with each other. The matrix of LEDs is usually driven by a lighting switch and is therefore subject to a potential difference between a power supply terminal and a terminal of the lighting switch.
A lighting branch is typically understood to mean one or more lighting sources connected in series with each other. A lighting branch is usually driven by a lighting switch and is therefore subject to a potential difference between a power supply terminal and a terminal of the lighting switch.
In the continuation of the description, for simplicity's sake, a lighting branch will be understood not only as one or more lighting sources connected in series with each other, therefore crossed by the same power supply current, but also as the lighting sources belonging to the same column of an LED matrix.
The solutions adopted up till now to overcome such drawback is therefore that of using matrices with two rows of LEDs, instead of the three row LED matrices usually used, or lighting branches with two LEDs in series, instead of lighting branches with three LEDs in series.
This means that, for the same number of LEDs, a circuit needs to be designed with a greater number of columns of the LED matrix or of lighting branches connected in parallel to each other. Since in a current stabilised driver circuit a lighting branch always absorbs the same current, regardless of the number of LEDs, increasing the number of columns or lighting branches in parallel means increasing the current absorbed by the circuit.
For example, for the same lighting sources, passing from a three row matrix to a two row matrix means absorbing 50% more current and thereby dissipating 50% more power.